1. Field of the Invention
The invention is a type of tool for use in turning fastening devices. The fastening devices relevant could be screws, whether machine, wood, or sheet metal type, bolts, cam-lock fasteners, or any other device which is operated by introducing torque at one end. The tool can be directly hand driven, turned by a hand wrench, or turned by an impact wrench.
2. Description of the Related Art
The typical screw type slotted head fastener in use is driven by introducing a torque to a slot in the head of the screw by use of a screw driver. The same method is used whether the screw is driven clockwise or counterclockwise, whether tightening or loosening. The most troublesome faults with this method of driving screws are that the screw driver tends to come out of the slot under high torque and that the outer edges of the slot tend to chip or deform when slippage occurs or when repetitive installation and removal are necessary.
The typical screw driver has a tapered blade which fits into the screw slot, which, conversely, has parallel faces. The result of this combination is that the end of the blade is narrow by comparison with the width of the slot, allowing the screw driver axis to be at an angle with the axis of the screw. In addition, the contact between the screw driver and the screw is made at the top edge of the slot, where the screw contacts the tapered edge of the screw driver blade at two points. When torque is applied to the screw driver, the axial misalignment and point contact cause one component of the force applied to push the screw driver out of the slot. This is the first fault alluded to above. The second fault is related. When the screw driver slips out of the slot because of this mismatch of tapered and parallel surfaces, the top edge of the slot frequently chips or burrs because the point loading exceeds the material strength just prior to the blade clearing the slot. Further, even if wholesale failure of the material at the top edge of the slot does not occur, repeated application of point loading at the top of the slot frequently causes abrasion or creep of the material, resulting in a slot with rounded edges which is no longer capable of transmitting torque to the screw.
The prior art exhibits several attempts to alleviate these problems. The most relevant of these attempts are described below, but none are as effective at solving the problems as the present invention.
U.S. Pat. No. 3,923,088 exhibits a blade 20 with opposing concave surfaces 22 the purpose of which is to allow the lower edge 30 of the blade to bite into the screw slot faces to resist the tendency of the blade to leave the slot. This configuration removes material from the blade, causing it to be weaker than the present invention. Because of the complicated shape, manufacture of the screw driver would also be much more difficult. If failure of the blade occurs, the average user would not be able to restore it to its original shape.
U.S. Pat. No. 3,897,812 exhibits a similar configuration with complicated contours which suffers from the same drawbacks.
U.S. Pat. No. 3,405,748 shows a straight bit 5 with parallel surfaces. Its torsion tube construction requires that for a given bit width, the shank must be considerably smaller and weaker than that of the present invention, given the same material of manufacture. It also will require a greater number of more difficult manufacturing operations, and the tube would be far more difficult to clean, a feature which is anathema to the precision work in which such tools are frequently used.
U.S. Pat. No. 1,479,506 shows a blade with concave surfaces like those previously discussed, with the same drawbacks. U.S. Pat. Nos. 4,105,056 and 4,311,071 exhibit blades with thin sections in the center which will suffer from weakness compared to the present invention and which will be more difficult to manufacture and impossible to repair.
U.S. Design Pat. Nos. Des. 112,592 and Des. 229,475 show apparently flat bits which are also difficult to manufacture and impossible to repair.
An article by Lee L. Dodds entitled "Justice for the Lowly Screwdriver" from the Aug. 26, 1946 issue of American Machinist magazine discloses a method of forming a screwdriver having a bit having parallel opposing surfaces. However, the screwdriver formed by the method disclosed therein suffers from two disadvantages: (1) the thickness of the bit is too thin, causing the tip to easily break when subjected to stress; and (2) there is no blade wedge, which results in the screwdriver being weaker than a standard, commercially available screwdriver. The further out one goes to apply torque, the more leverage there is. The inventor of the present invention took a commercially available screwdriver and modified it according to the American Machinist article and it broke the first time it was used.
Simpson et al., U.S. Pat. No. 4,878,406, discloses a screwdriver which has a bit having parallel faces and having a height approximately equal to its thickness. Because the height of the bit is approximately equal to its thickness, the bit can only engage a relatively small range of screw slots--the bit cannot fit in screw slots narrower than it, and in screw slots having a thickness greater than that of the bit, the screw will be contacted further up on the wedge-shaped blade, and thus no stress will be put on the bit (also, the edges of the screw slot will not be contacted by the parallel faces of the bit--thus, there is no advantage to using that screwdriver over conventional screwdrivers in larger screw slots).
Weible et al., U.S. Pat. No. 5,001,948, discloses a screwdriver having a bit which has parallel faces and is shaped to fit round screws, the screwdriver blade having increased thickness at areas which the patent describes as being high-stress areas. However, as with the '406 patent, the bit will not contact the sides of a screw slot when inserted into a screw slot which is thicker than the bit.